Performed a heating pm on an oil furnace. Brushed and vacuumed. Replaced the oil filter, nozzle and strainer. Started and checked the safeties. Performed a combustion test. 11.1% o2, 23 ppm, 82.6% efficient System is operating properly. While performing the combustion test, the O2 increased from 10.5% to 11.1% when the blower turned on. When there is an increase, that is a sign that a crack is forming. We can monitor it at our next visit.